The present invention relates to a golf ball including a core and a cover covering the core, and more particularly, to a multi-layered golf ball including a triple core.
In general, golf balls are called according to structure thereof and are classified into one-piece golf balls having only a single layer, two-piece golf balls having a single core and a single cover, three-piece golf balls having a single core and a double-layered cover, three-piece golf balls having a dual core and a single cover, four-piece golf balls having a single core and a triple-layered cover, four-piece golf balls having a dual core and a double-layered cover, four-piece golf balls having a triple core and a single cover, five-piece golf balls having a single core and a quadruple-layered cover, five-piece golf balls having a dual core and a triple-layered cover, five-piece golf balls having a triple core and a double-layered cover, six-piece golf balls having a dual core and a quadruple-layered cover, six-piece golf balls having a triple core and a triple-layered cover, and the like. Various golf balls described above may be manufactured, but commercially available golf balls developed to date are one-piece golf balls, two-piece golf balls, three-piece golf balls (two types of three-piece golf balls, i.e., three-piece golf balls having a single core and a double-layered cover, and three-piece golf balls having a dual core and a single cover), four-piece golf balls (two types of four-piece golf balls including a dual core and a double-layered cover, and four-piece golf balls including a single core and a triple-layered cover), five-piece golf balls having a single core and a quadruple-layered cover, five-piece golf balls having a dual core and a triple-layered cover and the like.
The term “core” herein used plays a key role in obtaining the carry distance of a golf ball by generating a strong repulsive elasticity when hit by a golf club. The core is generally manufactured by cross-linking a cis-1,4-polybutadiene rubber with α,β-ethylenically unsaturated carboxylic acid or a metallic salt thereof and additives such as a filler for cross-linking and curing the rubber. The core occupies the largest volume in the golf ball and is distinguished from the cover protecting the core from breaking, involved in flight characteristic, and having a relatively small volume.
In this cover, a cover layer which contacts an inside of the outmost cover having dimples, directly contacts the rubber layer having high repulsive elasticity and protects the cover as a separate layer is referred to as an inner cover. When a plurality of inner covers are present, the inner covers are for example classified into a first inner cover, a second inner cover and the like. These covers are formed by using a polymer material having a lower repulsive elasticity than the material used for the core, while having an appropriate elasticity and strong physical properties, such as an ionomer resin, polyester, a polyethylester copolymer, a thermosetting or thermoplastic polyurethane, a polyamide resin, a thermoplastic rubber, and the like.
The reason for presence of various types of golf balls as described above is as follows. For example, one-piece golf balls have a single element and thus inevitably have a single value of specific gravity, back spin performance, compression indicating a hardness level of balls and the like. For this reason, each physical property of produced golf balls is also represented as a single value. However, compression, back spin, trajectory, carry distance, feeling and the like of two-piece golf balls are changed according to specific gravity, hardness, and the softness of cores, and material, thickness, and the hardness of covers, so that golf balls having various functions are obtained. Similarly, three-piece, four-piece, five-piece and six-piece golf balls and the like having many functions and advantages and disadvantages may also be manufactured. The reason for this is that golf balls suitable for individual golfers are changed according to the capability of the golfers. When golf balls suitable for use by all golfers are present, only one golf ball will be present in the world. In this respect, the structure and material of a core that implements key function of golf balls, such as carry distance, back spin performance and the feeling are considerably essential. In a case of a single core, only one type of function is obtained from the core, but in a case of a dual or triple core, the golf balls have various different functions. For this reason, structure of the core is considerably essential. For example, an inner core is soft and an outer layer is hard with respect to the dual core that called as ‘hard outside and soft inside type’ golf balls, have reduced back spin obtained by a loft angle of a club head due to low resistance for overcoming and enduring a strong hit force from the center when hit by a long club such as a driver or long iron, and have bad directionality and short carry distance by the compensation force is lower when the golf balls glanced off the golf club. However, run is increased. Conversely, an inner core is hard and an outer layer is soft that called as ‘soft outside and hard inside type’ golf balls, have increased back spin due to high resistance for overcoming and enduring a strong hit force from the center when hit by the long club, and have good directionality and long carry distance due to high compensation force when the golf balls glanced off the club face. However, the run is slightly decreased.
The size of a dual core is also considerably essential. ‘Hard outside and soft inside type’ cores are easily broken, and have greatly decreased carry distance and bad directionality when the size of the soft inner core is greater than the size of the hard outer layer in which the size is converted to a length from the center of cores. However, golf balls become entirely excessively hard when the size of outer layer is greatly increased in order to prevent breakage. For this reason, control over the size of cores is considerably essential.
Conversely, soft-outside hard-inside type cores are not easily broken, and have increased carry distance and good directionality when the size of a hard inner core is greater than the size of the soft outer layer in which the size is converted in terms of length from the center of cores. However, with respect to this thickness, an absolute value of the minimum size of the outer layer containing a polybutadiene rubber as a main ingredient is necessarily required. The reason for this is that the outer layer may be broken by strong impact when the thickness is about 2 mm or less. Accordingly, a ratio of an outer layer size and an inner core size determined in consideration of the minimum thickness required for the outer layer is necessary. The feeling becomes harder when the inner core is larger than the outer layer. When the outer layer is excessively increased in size in order to overcome the hardness and provide softness, golf balls having defects of decreased carry distance and bad directionality are obtained. For this reason, size control is a considerably essential factor.
Meanwhile, specific gravity, which is an essential factor in a dual core, is closely related to driving property. In particular, difference in specific gravity between the inner core and the outer layer is a major factor in amount of spin of balls during flight or amount of spin of balls after landing. In accordance with regulations on weight and size by the United States Golf Association (USGA), and the Royal and Ancient Golf Club (R&A), the entire golf ball has a specific gravity of about 1.1288 g/cm3. A cis-1,4-polybutadiene rubber (hereinafter, referred to as polybutadiene) occupies the greatest volume among materials constituting the core of the golf balls and this rubber has a specific gravity of 0.91 g/cm3 and is thus light. Accordingly, heavy fillers other than reagents such as co-crosslinking agents directly participating in cross-linking reaction should be additionally added. Accordingly, specific gravities of respective regions are controlled by controlling difference in amounts of reagents used for the cross-linking reaction to control hardness of the inner core and the outer layer, or amounts of added fillers. Balls are easily broken when a great amount of fillers is used. For this reason, amounts of used fillers are limited. When the specific gravity of inner core is greater than that of the outer layer hitting the golf balls named as ‘soft-outside hard-inside type’ by a driver, a single axis is formed immediately after hitting and an initial spin of golf balls which fly while spinning based on the axis is increased and the flight trajectory get higher and carry distance are increased, but the run is decreased and golfers feel slightly light upon hitting. Conversely, the specific gravity of the outer layer is greater than that of the inner core, the flight trajectory gets lower, but the run is increased and golfers feel heavy when hitting by a club.
Other essential factors of the dual core are distribution of hardness in respective regions and compression indicating the overall hardness of golf balls determined by hardness. In general, a dual core is produced by first forming an inner core, covering the inner core with an outer layer by an appropriate method, and performing cross-linkage. Here, hardness is represented by a single representative value, but hardness distribution is changed, for example, the hardness is uniform from the center to the outside of the core, or is gradually increased, or increased and then decreased by control the mold temperature during molding in accordance with the kind of organic peroxide participating in cross-linking. The reason is the co-crosslinking agent such as α,β-ethylenically unsaturated carboxylic acid or a metallic salt thereof is partially gasified and reacted with polybutadiene based on metal particles in the mixture when cross-linking at a high temperature. The co-crosslinking agent may be partially not cross-linked and be volatilized, or may remain as a homopolymer formed by homopolymerization. In addition, some organic peroxides used as a polymerization initiator are decomposed at high temperatures and the other organic peroxides are decomposed at relatively lower temperatures. Accordingly, various forms of hardness distribution are obtained. Thus, Shore D hardness is not uniformity represented by a single value, and various distributions of Shore D hardness may be represented in the inner core and various distributions of Shore D hardness may also be represented in the outer layer. Although various hardness distributions are obtained, compression (here, this value means a value measured by an Atti compression tester) indicating the overall hardness of golf balls may be represented by a single value and the properties of golf balls having a dual core may be greatly diverse even though they have a constant compression. The reason is the compression represented by a single value, but golf balls having various properties are manufactured according to size, hardness distribution, specific gravity and the like of the inner core and the outer layer. The feeling of golf balls having an identical compression that alters according to materials constituting the inner core and the outer layer.